1. Field of the Invention
This invention relates to satellite position control systems. More specifically, this invention relates to earth sensor pulse processors for use on satellites.
While the present invention is described herein with reference to particular embodiments, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and the teachings of the present invention may recognize the advantages thereof in applications other than those mentioned above.
2. Description of the Prior Art
The position and/or orientation of a satellite or other spacecraft relative to the earth, the sun, the moon, or other celestial bodies is often critical to the success of its mission. To maintain a proper attitude, a satellite may be equipped with means for sensing its orientation relative to a frame of reference. A horizon sensor is such a sensing means.
As shown in U.S. Pat. No. 3,920,994 issued to the applicant in 1975, a horizon sensor typically includes optics and an infrared detector which generates an output whenever a radiant body is in the scanned path. This output may activate motive means to correct the satellite's orientation.
Spurious noise from other radiant bodies may trigger the infrared detector to generate a false output. To solve this problem, pulse processors have been developed which typically employ either a fixed threshold or a peak detector with an adaptive threshold to eliminate such noise effects.
A fixed threshold detector rejects input signals below a predetermined threshold. The threshold may, for example, be one-half the expected amplitude of the input pulse. Unfortunately, the amplitude of the input pulse may vary due to the latitude of the satellite relative to the radiant body, a change in the distance of the satellite from the radiant body, variations in the temperature of the radiant body, degradation of the optics associated with the infrared detector, or degradation of the infrared detector itself.
Since the amplitude of the input signal varies, it is desirable to vary the threshold in response to variations in the input pulse. Thus, it is an object of the present invention to provide a horizon sensor pulse processor with an adaptive threshold.
Peak detectors such as disclosed by Savoca (U.S. Pat. No. 3,457,410) typically differentiate the input signal to determine the peak amplitude. The value thus obtained is then used to set a subsequent threshold. In this sense, peak detectors are adaptive. Unfortunately, a peak detector can become a sun detector in that the input signal may be so great when the sun is detected that the threshold will automatically be set too high for signals corresponding to the earth.
Another disadvantage associated with peak detectors arises in the detection of long or wide pulses which are subject to droop. A peak detector may set the threshold so high as to detect only a portion of such a wide pulse, thus causing an erroneous indication as to the orientation of the satellite.
Thus, it is a still further object of the present invention to provide an adaptive comparator capable of comparing signals from the horizon sensor to an adapted threshold so as to discriminate signals from targets having varying degrees of radiation intensity. It is yet another object of the present invention to provide and adaptive comparator which can accurately detect wide pulses which may be subject to droop.